People toiling against their will in factories, farms and restaurants in Britain today are "slaves just as much as any slave ever was", Jeremy Corbyn has said.
The Labour leader called on every council in the UK to sign a local government charter against the practice at Islington town hall on Friday afternoon.
The document pledges that none of the more than £40 billion spent on goods and services annually by UK councils should enrich traffickers and slave drivers.
Speaking after meeting victims of the practice, Mr Corbyn said: "So many modern slaves are working in this country and people prefer not to know about it.
"You see on farms, in small factories around the country there are people who never get to the front of the building... they are slaves just as much as any slave ever was."
The document, produced by the Co-operative Party, contains a series of commitments to ensure councils' supply chains are not tainted by inadvertently funding exploitation.
These include challenging "abnormally low-cost tenders"; publicising whistle-blowing systems and cancelling deals for contractors who do not fully comply with the Modern Slavery Act 2015.
Councillors from Islington, Oxford, Stevenage, Lambeth and Waltham Forest signed the pledge - with the aim that every UK council follow.
Mr Corbyn added: "When the government in the 1960s, amid great controversy, introduced the Race Relations Act, it changed the atmosphere and debate because it was a public declaration of 'we're not going to accept it'.
"This is our declaration that we're not going to accept slavery within our society."
Modern slavery encompasses a range of criminality including servitude, forced labour and human trafficking.
Home Office figures suggest there are up to 13,000 potential victims in the UK, although anti-slavery commissioner Kevin Hyland has described the 2014 estimate as "far too modest".
In December, a watchdog warned efforts to tackle modern slavery will fall short without a major improvement in the Government's understanding of the crime.
Data collected by the Home Office is "variable", meaning it has an incomplete picture of the perpetrators and victims, the National Audit Office (NAO) found.
It also concluded that oversight of victim support is inadequate, and said few cases have led to prosecution.